qcow2_format.py: collect fields to dump in JSON format

qcow2_format.py: collect fields to dump in JSON format

As __dict__ is being extended with class members we do not want to
print, add the to_json() method to classes that returns a json-dumpable
object with desired fields and their values. Extend it in subclass when
necessary to print the final dictionary in the JSON output which
follows.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Message-Id: <1596742557-320265-10-git-send-email-andrey.shinkevich@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>